Implantable cardiac stimulating devices are devices that are adapted to be implanted within the body of a patient so that therapeutic electrical stimulation can be provided to the patient's heart to regulate heart function. These types of devices include well known pacemakers or implantable cardioverter-defibrillators (ICDs) or devices that include the functionality of both a pacemaker and an ICD. Typically, these devices consist of a control unit having a microprocessor and one or more leads that are adapted to be positioned adjacent the wall of the heart. The control unit generally receives sensory input about the function of the heart and, when the input is indicative of a heart arrhythmia, the control unit then provides an appropriate therapeutic electrical stimulation to the heart via the leads. The therapeutic electrical stimulation can, for example, consist of a low voltage pacing pulse to ensure that the heart is beating correctly or can include a high voltage waveform that is adapted to terminate a particular form of arrhythmia, such as fibrillation.
One particular difficulty with implantable cardiac stimulating devices is that the lead that is adapted to provide the electrical stimulation to the heart can become damaged. In many instances, the leads are implanted into the chambers of the heart. In this environment, the leads are continuously subjected to pressures as a result of the beating of the heart. Over time, the leads can become damaged or even broken such that the delivery of the therapeutic electrical stimulation can be hampered or interrupted.
This can be a particular problem with implantable cardiac stimulating devices that are adapted to terminate more serious forms of arrhythmia. For example, if the implantable cardiac stimulating device is adapted to recognize and provide a therapeutic shock to the heart upon the occurrence of ventricular fibrillation, a broken or damage lead may result in the implantable cardiac stimulating device being unable to provide this waveform. In this case, the ventricular fibrillation may not be corrected and the patient may die. Consequently, it is recognized that the status of the leads that provide the electrical stimulation to the heart must be periodically checked to ensure that the leads are still capable of providing the therapeutic stimulation to the heart.
In fact, it is often recommended that patients who have implanted ICD's periodically have chest x-rays taken so that the status of the leads implanted within the heart can be ascertained. These x-rays can reveal whether a lead is broken, such that the ICD may be unable to provide a therapeutic shock to the heart when the heart experiences a serious arrhythmia. However, using x-rays to assess lead status has several difficulties.
For example, while x-rays may be able to reveal some broken leads, the x-rays may not be able to reveal damage to the leads that would increase the impedance of the lead. An increase in the impedance of the lead may result in the magnitude of the shock being delivered to the heart being degraded such that the shock may be unable to halt the life threatening arrhythmia. Moreover, periodically x-raying patients is expensive and time consuming. Consequently, the x-rays may only be taken at relative long time intervals and the damage to the lead may actually occur in between x-rays. Consequently, some damaged leads may not be detected prior to the lead being called upon to deliver a therapeutic stimulation to the heart to correct a heart abnormality.
To address these particular problems, some implantable cardiac devices of the prior art have instituted procedures whereby the impedance of the leads are periodically measured. Once such example is provided by U.S. Pat. No. 5,549,646 to Katz et al. The device disclosed in this patent included an impedance measurement circuit that has a voltage source which applies a voltage to the ICD leads so that an impedance measurement of the ICD leads can be obtained. The impedance measurement of the ICD leads can then be compared to a reference value to determine whether the lead impedance has exceeded a predetermined amount. One difficulty of the impedance measurement circuit disclosed in U.S. Pat. No. 5,549,646 is that additional circuitry must be provided to the device in order to obtain the impedance measurement.
Moreover, the impedance measurement of the leads is compared to a reference value and this reference value is generally greater than the normal impedance of the lead. Hence, the circuit disclosed in U.S. Pat. No. 5,549,646 is only capable of providing an indication that the lead has been significantly damaged but generally does not provide any indication of small or moderate damage to the lead which results in a small or moderate increase in the lead impedance. Again, moderate damage to the lead may still result in a decrease in the amplitude of a stimulating electrical shock that is to be delivered to the heart which can result in inefficient operation of the implantable cardiac stimulating device or even result in the applied shock being unable to terminate a life threatening arrhythmia.
Another example of a prior art implantable cardiac stimulating device that incorporated circuitry for measuring lead impedance is U.S. Pat. No. 5,755,742 to Schuelke et al. This patent discloses a system whereby the impedance of ICD coils is measured as a result of low voltage pacing pulses being delivered from pacing leads that are implanted within the heart. The pacing pulses are received by the ICD coils such that the impedance of the ICD coil can then be measured. The measured ICD coil impedance is then compared to maximum and minimum impedance values to determine if the ICD coil has experienced a particular problem. However, the device disclosed in U.S. Pat. No. 5,755,742 compares the resulting impedance measurement to maximum and minimum values which are less capable of providing an indication that the lead has suffered small or moderate damage. This problem is compounded by the fact that there is generally not a linear relationship between an impedance of an implanted coil measured at a low voltage and the corresponding impedance of the coil that would occur when a high voltage therapeutic electrical shock is being applied to the heart. Consequently, it is difficult to set minimum and maximum reference values for impedances of leads measured using low voltage pulses that will accurately provide an indication as to whether the lead has experienced some sort of damage that may hinder future delivery of therapeutic waveforms.
Consequently, there is a need for a system which would be capable of measuring the impedance of leads using low voltage pulses that is capable of providing an indication that the lead has experienced damage which could potentially jeopardize the successfully delivery of therapeutic waveforms to the heart to terminate an arrhythmia. To this end, there is a need for a system that is capable of measuring the impedance of the lead at a low voltage and then determining whether the measured impedance is indicative of a possible problem with the lead that requires further analysis.